This invention relates to a novel and useful process for preparing the decapeptide L-Pyr-L-His-L-Trp-L-Ser-L-Tyr-D-Trp-L-Leu-L-Arg-L-Pro-Gly-NH.sub.2 ([D-Trp.sup.6 ]-LH-RH) and to intermediates therefor.
The naturally occurring decapeptide, Pyr-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH.sub.2 (LH-RH), was isolated and its structure was elucidated by A. V. Schally et al., Biochem. Biophys. Res. Commun., 43, 393 and 1334 (1971). [D-Trp.sup.6 ]-LH-RH has been established to be a useful compound for causing release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in a mammal by A. V. Schally and D. H. Coy, U.S. Pat. No. 4,010,125, Mar. 1, 1977, incorporated herein by reference, and D. H. Coy et al., J. Med. Chem., 19, 423 (1976). The above cited U.S. Pat. No. 4,010,125 and the J. Med. Chem. article describe a process for preparing [D-Trp.sup.6 ]-LH-RH wherein the decapeptide is prepared by solid-phase methodology using protecting groups on the side chain functional groups. Although the solid-phase is useful for preparing small and experimental quantities of the decapeptide, the yield thereby obtained is low and the method of purifying is time consuming and expensive so that use of this method as a commercial process for the production of a valuable drug is economically prohibitive. Furthermore, a rather large number of preparations of LH-RH and its analogs or derivatives have been reported, for example, H. U. Immer et al., U.S. Pat. No. 3,835,108, issued Sept. 10, 1974. However, it is an accepted fact with those skilled in the art of synthetic peptide chemistry that there is no obvious or analogous method for a good preparation of a decapeptide. The latter statement was confirmed in a realistic fashion when an attempt to prepare the object compound of this invention by an analogous process to that described in U.S. Pat. No. 3,835,108 failed because of extremely low yields. The successful preparation of the peptide or a derivative thereof cannot be designed ahead of time. Using a known preparation of even a close derivative does not guarantee a reasonable synthesis of the desired peptide. The methods of coupling, selection of protecting groups and fragments to be used must be determined by the peptide chemist in each preparation.
It has now been found that the process of this invention produces [D-Trp.sup.6 ]-LH-RH in commercial quantities at an economically acceptable cost. Surprisingly, this process involves in the final step a coupling of an unprotected tripeptide with an unprotected heptapeptide. A coupling of a tripeptide with a heptapeptide to prepare LH-RH is described by H. Sievertsson et al., Biochem. Biophys. Res. Commun., 44, 1566 (1971). However, the latter preparation uses a different coupling method and also with protecting groups present during the final coupling. Furthermore, use of the same protecting groups as used in the latter report for the preparation of [D-Trp.sup.6 ]-LH-RH would not result in a successful preparation because of complications arising from the presence of the two tryptophane residues.